A rational goal of therapeutics is to define the dose-response relationship. This can be done by modelling the dose-concentration and concentration-response relationships, i.e., pharmacokinetics and pharmacodynamics. A combined pharmacokinetic and pharmacodynamic approach is proposed. Models will be developed and tested and a complete system developed to facilitate this kind of modelling using the PROPHET computer system. The pharmacokinetic and pharmacodynamic interactions between digoxin and quinidine will be studied in dogs. The drug response will be based on changes in cardiac repolarization. A new method for computer-assisted measurement of the T-wave complex of the ECG is proposed. The pharmacodynamic interaction between digoxin and quinidine will be evaluated in man.